With the current “wave” of the commercialization of Internet usage, and the increasing usage of multimedia applications, the traffic demand is seen as a phenomenon which is exploding today. The response from the wired world is the emergence of Dense Wavelength Division Multiplexing (DWDM) technology, which can increase link throughput by one magnitude or more. That response leaves the last mile especially the wireless link as a potential bottleneck. As a consequence, the task of searching for a method for increasing the wireless link and its related network capacity is urgent.
This is why the spectrum efficiency becomes the first consideration for the spectrum license auction in most countries around the world. Whoever has the solution offering higher efficient use of the spectrum will win the bid. The traditional way of increasing the traffic throughput can be categorized into two main fundamental classes, one is deterministic, and the other is statistical.
For the deterministic class, there is a frequency reuse plan or cellular concept, which works in both the frequency domain and space domain. High frequency reuse can be achieved by reducing cell size, however, that means increasing the number of expensive Base Transceiver Stations (STS). This notion of “Pico cell” (approximately 100 meters in radius) has failed completely in cellular system deployment due to its high cost. Such an impractical system would include CT2+ (Cordless telephone Phase 2).
Another method is using high order modulation such as Quadrature Amplitude Modulation (QAM) and high efficiency coding schemes such as Turbo code [IEEE802.16.1 Standard]. These methods can both be considered as time domain methods.
The penalty of using n-QAM (n=4, 16, 32, 64, 128 . . . ) is that a higher Signal to Noise Ratio (SNR) is required. In other words, a transmitter with higher transmitting power and a receiver with a lower noise figure are needed. This makes the design of the system throughout the transmitter to receiver more complicated. For example, designing a linear Power Amplifier (PA) for 64 QAM for the Customer Premise Equipment (CPE) is already very challenging, bearing in mind that the PA is the most expensive component in a transmitter design.
The other limiting factor that prevents very high order QAM from being applied in wireless systems is that the cost of driving down the phase noise in the Phase Locked Loop (PLL) is still high. Higher QAM needs lower noise PLL.
As a consequence, some other researchers are using the concatenated Forward Error Coding (FEC) schemes such as Turbo code to approach the channel capacity limit, i.e. Shannon capacity limit. With a complicated soft iterative decoding algorithm, the limit can be approached to within less than 0.1 dB ranges. By applying different puncturing patterns, the different coding rate k/n can be achieved in practice, where k is the number of user information, and n is the total number of bits coded. Apparently this approach has reached its limit.
There is not much that can be done in the time domain processing, and most researchers have moved on to the space domain, studying the possibility of using time-space coding to take the advantage of the antenna diversities—space resource. This approach is promising, however, the cost is dependent on the increased number of antennas and the additional computation intensive processing [IEEE WCNC paper L2.1 by David Tse et al] in order to make use of multi-paths that exist in a certain environment for a certain frequency range.
This method is arranged on the evolution path of the Othogononal Frequency Division Multiple Access (OFDM). But the price of radio and antenna will prevent such a scheme from being deployed.
Therefore, as is apparent from above discussions, there are challenging problems in advancing deterministic methods.
For the statistical method, one type is the Bandwidth on Demand (BoD) strategy based on the Time Division Multiple Access (TDMA) method. Apparently, this scheme, which works in the time domain, has also reached its maximum capacity. Many-advanced methods have been already proposed [ETSI DV-RCS latest revision for Media Access Control (MAC) section]. Most of above methods work at lower layer with a deterministic approach.
An example of a system which involves lower layer hardware assisting higher layer real time software processing, and vice versa is an optical device, which is able to directly process transport layer Internet Protocol (IP) headers without intervention of electrical signals at all.
Although the service provider is not able to charge the end customer more for best effort traffic, however, the quality of service experienced largely depends on a “bad” experience when the end-customer suffers excessively long congestion periods. As a consequence, to minimize congestion experiences people have had with cable modems is important for a new service to gain a “good” reputation over the time.
The practical methods for coping with congestion can be placed in two in 2 different categories. One is the host centric, window and feedback based method used in Internet Protocol (IP), and the other is the switch centric, rate and reservation-based method employed in Asynchronous Transfer Mode (ATM).
Most (if not all) congestion control methods used in the past like Transmission Control Protocol (TCP) for IP or Available Bit Rate (ABR) for ATM are to throttle the traffic source.
Broadband Wireless Access (BWA) services are commonly being deployed with Multi-channel Multi-point Distribution System (MMDS) or Local Multi-point Distribution System (LMDS) methods [IEEE Computer July 2000 by Sixto Ortiz Jr.]. For the case of using the LMDS method, the microwave frequency employed is around Ka band, i.e. 11 GHZ to 60 GHz. At this frequency range, rain causes severe attenuation of the signal.
For example, for the rain region K, to achieve a reasonably high availability of 99.99% around the year for the horizontal polarized frequency of 43.5 GHz the radio will need a 28 dB link margin [ETSI BRAN#20 contribution BRAN20d028 by Paolo Agabio]. This issue is one of the key problems encountered in the design of such a communication system.
In practice, the rain margin for some radios [e.g. DragonLink or DragonFire from DragonWave] can be set as high as 25 dB—and it is only used once in a while (53 minutes downtime per year to meet 99.99% availability). The automatic Power Control (APC) is usually controlled by a rain event, i.e. when there is rain on the radio signal path between the CPE and the base station, the power at the transmitter is increased to compensate the rain attenuation.